Covalent modification of histones, such as acetylation, methylation, phosphorylation, and ubiquitylation, are essential regulators of chromatin structure and function. Defects in the regulation of these modifications have causal roles in numerous developmental disorders and diseases. However, the mechanisms that regulate histone-modifying activities are not well understood. In addition, apart from a few well-known examples, it is not clear how the cellular machinery interprets these modifications. The PWWP domain is similar to Chromo, Tudor, and MBT domains, which are well known for their ability to recognize methylated histones to regulate diverse cellular processes. However, the function of the PWWP domain is still a mystery. Our preliminary studies demonstrated that PWWP domain proteins directly interact with histones and form complexes with histone-modifying activities. I therefore hypothesize that PWWP domain proteins might recognize modified chromatin components to regulate associated enzymes. I will characterize the functions of PWWP domain proteins in fission yeast to gain mechanistic insights into their roles in regulating chromatin structure and function. The specific aims are designed to (a) determine the histone modification that interacts with each PWWP domain protein with pull-down assays and chromatin immunoprecipitation (ChIP) analysis; (b) characterize PWWP domain protein complexes identified by affinity purification of epitope-tagged PWWP domain proteins to elucidate the contribution of each protein to the function of these complexes; (c) analyze the cellular functions of PWWP domain proteins in chromatin based processes, with the goal of directly linking phenotypes to specific histone modifications. Mutations in many PWWP domain-containing proteins are intimately linked to human diseases. For example, a single amino acid alteration in the PWWP domain of Dnmt3b is responsible for ICF (immunodeficiency, centromeric instability, and facial anomalies) syndrome. A better understanding of the function of PWWP domain proteins may lead to novel therapeutic approaches to treat these diseases.